Process for carrying out chemical reactions in an electrochemical cell

ABSTRACT

This invention concerns a process in which gases or gas mixtures are reacted in the presence of an ion-conductive liquid in an electro-chemical cell. The cell has at least one anode ( 2 ) and at least one cathode ( 3 ) to which an external electrical constant potential is applied so that a direct current flows through the ion-conducting liquid. In the lower region of the cell a sump ( 4 ) of ion-conductive liquid is located into which the electrodes are partially immersed. At least 20% of the entire surface of at least one of the electrodes is located outside the sump in an upper region through which gas flows. This upper region of the cell is sprayed or irrigated with the ion-conductive liquid and the electrode surface at least partially wet. During this wetting process, the gas flows across the electrode surface.

This invention relates to a process of reacting a gas or gas mixture inthe presence of an ion-conducting liquid in an electrochemical cellcomprising at least two electrodes, namely at least one anode and atleast one cathode, where between the cathode and the anode an electricd.c. voltage is acting, which has been applied from the outside, and adirect current flows through the ion-conducting liquid.

Such process is described in the German Patent 195 04 920. Theelectrochemical cell contains an aqueous ammonium sulfide solution,which virtually completely covers the electrode surfaces. Gas containingfree oxygen gets in contact with the solution through a gas diffusioncathode, so that ammonium polysulfide is formed as product. In thisconnection it should be noted that the liquid level in the cell cannotbe chosen arbitrarily, as otherwise a disturbing leakage may occur.Furthermore, the current-voltage characteristic of the cell isunfavorably influenced by a high liquid level.

It is the object underlying the invention to perform the electrochemicalreaction of gases with liquids also in the presence of catalysts in aninexpensive way with a high consumption, safely and also at a highpressure. In accordance with the invention this is achieved in theabove-mentioned process in that in the lower portion of the cell a sumpof the ion-conducting liquid is provided, into which the electrodes areimmersed, that at least 20% of the entire surface of at least one of theelectrodes are disposed outside the sump in an upper portion throughwhich flows the gas or gas mixture, and that the upper portion issprinkled or sprayed with the ion-conducting liquid, and the electrodesurface is at least partly wetted while the gas or gas mixture flowsalong the electrode surfaces. In this way, different gases and liquidscan be reacted. Usually, the gas or gas mixture is oxidized or reducedin the process.

All or at least part of the electrodes are disposed vertically in thesump of the ion-conducting liquid, and this sump ensures the flow ofcurrent between the electrodes. In most cases, 20-95% of the entiresurface of at least one of the electrodes will be disposed above thesump. It is also possible that either the anode or the cathode iscompletely covered by the liquid of the sump. The electrodes may notonly have a plate-shaped or a cylindrical design, but one electrode mayalso be designed as current-conducting bed or stacked packing ofcurrent-conducting elements contacting each other. Such bed or packingmay additionally be coated with a catalyst.

Between the anode and the cathode of the cell a d.c. voltage is appliedfrom the outside, which may be chosen in a wide range. The voltagebetween adjacent anodes and cathodes may lie in the range between 0.01and 100 V, usually these voltages lie in the range from 0.1 to 10 V.

A large part of the entire electrode surface is disposed outside theliquid sump and is sprayed or sprinkled by the liquid serving aselectrolyte. At the same time, the gas conducted into the cell gets incontact with the surfaces of the electrodes, which are disposed outsidethe sump. In this connection it is not important in what direction thegas flows. The gas may first of all be introduced into the liquid sumpin the lower portion of the cell and flow upwards, or the gas isintroduced into the upper portion of the cell, without conducting itthrough the sump, to the sprayed or sprinkled electrodes. With the gas,one component for the reaction to be performed in the cell, for instanceoxygen or hydrogen, can be supplied. As gas, there can thus beintroduced air, O₂, H₂S, NH₃, SO₂, SO₃, or a synthesis gas mixture(CO+H₂) or also mixtures of these gases into the cell.

The ion-conducting liquid contained in the cell, which also serves aselectrolyte, will usually be an organic or inorganic solution or a melt.

The electrodes may consist of different materials, and they may beformed for instance from metal alloys, mixed oxides or be carbonaceous.When the electrode material itself has no catalytic effect, a catalystmay for instance be applied as coating on an electrically conductivesubstrate. In this way, both cathodes and anodes may be especiallydesigned for different reactions. It is also possible that theelectrodes are consumed during the reaction and act as what is calledsacrificial electrodes. When high-carbon electrodes are employed, it maybe expedient to make their surface hydrophobic, which is accomplished ina known manner by partly covering the surface withpolytetrafluoroethylene.

When it is desired to subdivide the cell into a plurality of reactionchambers with partial exchange of liquid, this can be achieved by meansof a diaphragm or also a plurality of diaphragms, which are porous andliquid-permeable in a manner known per se. A further possibility for thesubdivision is to use ion-selective membranes, which are likewise knownper se.

The desired product of the reaction in the cell may be contained in theliquid withdrawn from the cell or in the exhaust gas withdrawn or bothin the exhaust gas and in the liquid. The separation and concentrationof the product is then effected in a manner known per se.

The control of the desired reaction or reactions is effected forinstance by varying the supply of gas and/or liquid and also by the flowof current in the cell and the electric voltage applied from theoutside. Furthermore, the redox potential in the electrolyte sump can bemeasured and be used as control variable.

Embodiments of the process will be explained with reference to thedrawing, wherein:

FIG. 1 shows a first variant of the electrochemical cell in a schematicrepresentation,

FIG. 2 shows a second variant of the cell,

FIG. 3 shows a third variant of the cell,

FIG. 4 shows a horizontal section along line IV—IV of FIG. 3,

FIG. 5 shows the horizontal section through a cell similar to FIG. 3,

FIG. 6 shows a cell with bipolar electrodes, and

FIG. 7 shows a cell with a gas diffusion electrode.

In accordance with FIG. 1 the electrochemical cell is disposed in aliquid- and gas-tight housing 1 and comprises an anode 2 and a cathode3. The two electrodes are connected with an external d.c. voltage sourcenot represented here. In the lower portion of the cell a liquid sump 4is provided, whose liquid surface is indicated by a broken line 5. Theliquid serves as electrolyte, it is circulated in part and for thispurpose recirculated through the line 7, the pump 8, the return line 9and the distributor 10 and sprayed onto the electrodes from above. Partof the liquid is withdrawn as product through line 12, and fresh liquidis supplied to the circuit through line 13. A gas or gas mixture issupplied via line 15 and can first of all enter the sump 4, before itflows upwards between the sprayed electrodes, where the de- siredreaction takes place. Exhaust gas is removed from the housing 1 throughline 11. Depending on the type of reaction, this gas can likewise beregarded as product.

As can be seen in FIG. 1, only the lower part of the electrodes 2 and 3is disposed in the electrolyte sump 4, where a current can flow throughthis sump between the electrodes. At least 20% of the entire surface ofthe electrodes are disposed above the sump 4, and these surfaces are atleast partly wetted by the liquid droplets originating from thedistributor 10. At the same time, the gas or gas mixture coming fromline 15 flows upwards along the wetted electrode surfaces. In mostcases, 20 to 95% of the entire surface of the electrodes will bedisposed above the sump 4.

In the cell of FIG. 2 one of the electrodes, in this case the cathode 3a, is designed as liquid- and gas-permeable bed or packing, where theelements are in electrically conducting contact with each other. Theanode 2 is formed by a horizontal plate, which is completely disposed inthe sump 4. The surface 5 of the sump extends up to the lower portion ofthe cathode 3 a. The supply of gas is effected through line 15, and theremaining parts of the arrangement have the meaning already explained inconjunction with FIG. 1.

In the cell of FIG. 3, the anode is formed by a plurality of vertical,parallel plates 2 a, whose lower portion extends into the liquid sump 4.The cathode 3 is designed as horizontal plate disposed in the sump 4.The remaining parts of the arrangement of FIG. 3 have already beenexplained in conjunction with FIG. 1, and the circulating pump 8 wasomitted in FIG. 3 for simplification. In the horizontal section of FIG.4, the vertical anode plates 2 a can likewise be seen.

In contrast to FIG. 3 and 4, the anode can also be designed as aplurality of concentric cylinders 2 b, which are open at the bottom andat the top and are partly disposed in the electrolyte sump. Moreover,such cell can be designed in accordance with FIG. 3. In contrast to therepresentation of FIG. 3 to 6, the electrically positive pole can beprovided on the illustrated anode, and the negative pole can be pro-vided on the illustrated cathode, without otherwise changing the cell.

FIG. 6 shows a cell with bipolar electrodes, which may be designed asparallel, vertical plates and are disposed between the terminal anode 2and the terminal cathode 3. In contrast to this, bipolar electrodes mayalso be designed as concentric cylinders. The remaining parts of thearrangement in accordance with FIG. 6 have already been explained inconjunction with FIG. 1.

In accordance with FIG. 7, the housing 1 comprises a gas diffusioncathode 3 b, to which a liquid-free gas space 17 is associated. The gasis supplied through line 15 a and withdrawn through line 15 b. In thegas space 17 part of the gas flows through the porous structure of thegas diffusion cathode 3 b to get in contact with the electrolyte, whichis disposed in the sump 4 and is sprayed from the distributor 10. Insidethe structure of the cathode 3 b, gas and liquid thus get in contactwith each other, but without disturbing quantities of gas or liquidcompletely penetrating through the cathode structure. In a manner knownper se, the gas diffusion cathode 3 b may consist of a metal net and acarbon cloth attached thereto. Advantageously, the fibers of the carboncloth have been made hydrophobic at least in part, as is likewise known.

EXAMPLE 1

In the laboratory, an arrangement in accordance with FIG. 3 is employed,but the cathode 3 described there now becomes the anode. The anode isarranged horizontally and fully immersed in the sump 5, it consists of acircular disk of titanium expanded metal activated with platinum, thediameter is 100 mm, and the thickness is 1 mm. The cathode is formed by8 parallel, vertical plates having a height of 90 mm and a width of 50mm, which have a distance of 4 mm and are conductively connected witheach other. The cathode plates consist of titanium expanded metalactivated with platinum. The cathode plates are immersed in theelectrolyte sump for 20 mm. The container 1 is made of glass.

The cathode plates are sprinkled from above with an aqueous solution,which contains 5 g/l NaOH and 6.3 g/l Na₂SO₃ and has a temperature of50° C. Air is supplied through line 15, and the exhaust air of line 11is partly recirculated to line 15. The amount of recycle gas is 450Nl/h, fresh air is admixed to the recycle gas in an amount of 100 Nl/h.The supply of gas into the sump 4 is effect 10 mm below the liquid level5, the circulating amount of liquid is 4 l, and the object of theprocess is the oxidation of sulfite ions to form sulfate ions.

A first experiment was made with a current of 1A, for control purposes asecond experiment was made without current, and finally a thirdexperiment was performed completely without electrodes. After a periodof 2 hours for each experiment, the following amounts of sulfite ionswere oxidized to form sulfate ions:

1st experiment 66 wt% 2nd experiment 17 wt% 3rd experiment  5 wt%

EXAMPLE 2

In the laboratory, an apparatus in accordance with FIG. 2 is employed,where the anode 2 is formed by a circular graphite web with a diameterof 100 mm and a thickness of 25 mm, which extends horizontally in thesump 4 of the glass container 1. The cathode 3 a is formed by fourgraphite web layers lying one above the other with a total height of 100mm, where at the lower and upper end of the cathode a polypropylene netis disposed for stabilization. The cathode 3 a is immersed into the sump4 for 20 mm, and the diameter of the cathode like the inside diameter ofthe container 1 is 120 mm.

An aqueous solution of 4.2 g/l NaOH and 8.5 g/l Na₂SO₃ is recirculatedin an amount of 4 l and sprayed onto the cathode 3 a, and the supply ofgas is effected as in Example 1. Just as in Example 1, three differentexperiments are each performed for 2 hours, and the following resultswere obtained with respect to the amount of sulfite ions oxidized toform sulfate ions:

1st experiment (current 1A) 47 wt%  2nd experiment (currentless) 9 wt%3rd experiment (without electrodes) 3 wt%

What is claimed is:
 1. A process for reacting a gas or a gas mixturewith an ion-containing liquid in an electrochemical cell to obtain agaseous or liquid reaction product which comprises the steps of: (a)providing an electrochemical cell which comprises (1) at least twoelectrodes having a plank-shaped or cylindrical design, including atleast one anode and at least one cathode; (2) a d.c. power supply inelectrical contact with each of said electrodes, said d.c. power supplyapplied to each of said electrodes from outside of the cell; (3) aliquid and gas-tight housing surrounding said at least two electrodes;(4) a sump at the bottom of the electrochemical cell comprising theion-containing liquid in which the electrodes are immersed or partiallyimmersed; (5) a gas inlet for introducing the gas or gas mixturereactive with the ion-containing liquid in the electrochemical cell; (6)an upper portion of the electrolytic cell above the liquid line of theion-containing liquid in the sump wherein at least 20% of the entiresurface of the at least one electrode partially immersed in theion-containing liquid is disposed above the liquid line; (7) a gasoutlet for removing the gaseous reaction product from theelectrochemical cell; (8) an inlet for introducing fresh ion-containingliquid into the upper portion of the electrochemical cell; (9) an outletfor removing spent ion-containing liquid or the liquid reaction productfrom the sump of the electrolytic cell; (10) means for spraying aportion of the ion-containing liquid onto the at least 20% of the entiresurface of the electrode disposed above the sump in the upper portion ofthe electrolytic cell; and (11) a pump for recirculating theion-containing liquid from the sump to the means for spraying disposedabove the sump; (b) spraying the at least 20% of the entire surface ofthe at least one of the electrodes disposed above the liquid line of theion-containing liquid in the sump to wet the entire surface of theelectrode above the liquid line; (c) introducing a gas or gas mixturereactive with the ion-containing liquid into the sump containing theion-containing liquid; and (d) electrolytically reacting theion-containing liquid with the gas or gas mixture reactive with theion-containing liquid along the surface of the electrode above theliquid line to obtain a reaction product.
 2. The process defined inclaim 1 wherein according to step (d) the gas or gas mixture is oxidizedin contact with the ion-containing liquid and the electrodes above theliquid line of the ion-containing liquid in the sump.
 3. The processdefined in claim 1 wherein according to step (d) the gas or gas mixtureis reduced in contact with the ion-containing liquid and the electrodesabove the liquid line of the ion-containing liquid in the sump.
 4. Theprocess defined in claim 1 wherein according to step (a) 20 to 95% ofthe entire surface of the at least one electrode partially immersed inthe ion-containing liquid is disposed above the liquid line.
 5. Theprocess defined in claim 1 wherein according to step (a) the surface ofat least one of the electrodes immersed or partially immersed in theion-containing liquid is catalytically active.
 6. The process defined inclaim 1 wherein at least one of the electrodes is a gas diffusionelectrode, one side of which is porous and is in contact with the gas orgas mixture and the other side of which is sprayed with theion-containing liquid.